Multiple zone coil antenna with plural radial lobes

ABSTRACT

A low inductance coil antenna for a plasma reactor has multiple radial zones of plural conductor lobes extending radially from respective RF supply and ground rings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.61/886,853 filed Oct. 4, 2013 entitled PLASMA REACTOR WITH A MULTIPLEZONE COIL ANTENNA OF PLURAL LOBES, by Vladimir Knyazik, et al., andassigned to the present assignee.

BACKGROUND

1. Technical Field

An inductively coupled plasma source of a plasma reactor generallyincludes an antenna having plural conductive coils and an RF powergenerator coupled to the antenna.

2. Background Discussion

An inductively coupled plasma source is employed in many plasma reactorsfor performing plasma-enhanced processes, such as etching, on aworkpiece such as a semiconductor wafer. An inductively coupled plasmasource includes one or more RF-driven inductive coils to deliver powerto a plasma. The coil antenna may include two or more coils in order tocontrol radial distribution of plasma ion density. A typical coilantenna may be a flat spiral conductor or a cylindrically shaped spiralwinding. The inductance of such a coil antenna is typically very high,causing the voltage on the RF power terminal of the coil antenna to bevery high. As a result, it is often necessary to connect a capacitor inseries with the coil antenna in order to reduce the voltage on the RFpower terminal. One problem is that the introduction of such a seriescapacitor reduces the efficiency of the coil antenna.

Another problem with a typical inductively coupled plasma source is thatthe use of two or more coils in the antenna creates an M-shaped radialdistribution of plasma ion density, in which there are pronounced minimaat the wafer edge and at the wafer center. This problem arises frominteraction or mutual coupling between the different coils of theantenna.

SUMMARY

A coil antenna for a plasma reactor comprises an inner antenna and anouter antenna surrounding the inner antenna, wherein the outer antennacomprises: (1) plural conductive outer lobes extending in respectiveradial directions with respect to a central axis, each of the pluralconductive outer lobes comprising an elongate conductor having a firstouter end and a second outer end; and (2) a first outer connection tothe first outer ends of the plural conductive outer lobes and a secondouter connection to the second outer ends of the plural conductive outerlobes, the first and second outer connections being adapted for couplingto an RF power source. Further, the inner antenna comprises: (1) pluralconductive inner lobes extending in respective radial directions withrespect to the central axis, each of the plural conductive inner lobescomprising an elongate conductor having a first inner end and a secondinner end; and (2) a first inner connection to the first inner ends ofthe plural conductive inner lobes and a second inner connection to thesecond ends of the plural conductive lobes, the first and second innerconnections being adapted for coupling to an RF power source.

In one embodiment, respective ones of the plural outer conductive lobesextend radially from respective ones of the first and second outer ends;and respective ones of the plural inner conductive lobes extend radiallyfrom respective ones of the first and second inner ends.

In a further embodiment, the elongate conductor of each of the pluralconductive outer lobes and of the plural conductive inner lobes followsa curved path comprising a loop.

In a yet further embodiment, the first and second outer connections areseparated by a first gap distance and the first and second innerconnections are separated by a second gap distance; the elongateconductor of each of the plural conductive outer lobes has a thicknessalong the axis of symmetry less than the first gap distance; and, theelongate conductor of each of the plural conductive inner lobes has athickness along the axis of symmetry less than the second gap distance.In a related embodiment, adjacent ones of the plural conductive outerlobes at least partially overlie one another without contacting oneanother, and adjacent ones of the plural conductive inner lobes at leastpartially overlie one another without contacting one another.

In accordance with a further related embodiment, adjacent ones of theplural conductive inner lobes overlap one another, and a proportion ofoverlapping areas of the plural conductive inner lobes varies as a firstfunction of radial location; and adjacent ones of the plural conductiveouter lobes overlap one another, and a proportion of overlapping areasof the plural conductive outer lobes varies as a second function ofradial location. In yet a further embodiment, at least one of the firstand second functions compensates for a predetermined nonuniformity inradial distribution of plasma ion density in a processing zone adjacentthe coil antenna.

In accordance with one embodiment, the coil antenna further comprises: afirst RF power port coupled to the first and second inner connections; asecond RF power port coupled to the first and second outer connections;and, a controller capable of governing a ratio of power levels of thefirst and second RF power ports.

In accordance with an embodiment, the coil antenna further comprisesfirst respective variable capacitors connected in series betweenrespective ones of the elongate conductors of the plural conductiveinner lobes and one of the first and second inner connections. In arelated embodiment, one of the first and second inner connections isconnected to a return potential.

In accordance with another embodiment, the coil antenna furthercomprises second respective variable capacitors connected in seriesbetween respective ones of the elongate conductors of the pluralconductive outer lobes and one of the first and second outerconnections. In a related embodiment, one of the first and second outerconnections is connected to a return potential.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the presentinvention are attained can be understood in detail, a more particulardescription of the invention, briefly summarized above, may be had byreference to the embodiments thereof which are illustrated in theappended drawings. It is to be appreciated that certain well knownprocesses are not discussed herein in order to not obscure theinvention.

FIG. 1 is an elevational cut-away view of a plasma reactor embodying oneaspect.

FIG. 2 is an orthographic projection of a coil antenna of the plasmareactor of FIG. 1.

FIG. 3 is a plan view corresponding to FIG. 2.

FIG. 4 is an elevational view corresponding to FIG. 2.

FIG. 5 is a plan view corresponding to FIG. 3 illustrating certaingeometrical parameters of the coil antenna.

FIG. 5A depicts a modification of the embodiment of FIG. 5.

FIG. 6A is as graph depicting an M-shaped radial distribution of plasmaion density (solid line) and a corrected distribution of plasma iondensity (dashed line).

FIG. 6B is a graph depicting proportion of overlap between adjacentconductor lobes as a function of radial location, for transforming thesolid line distribution of FIG. 6A to the dashed line distribution ofFIG. 6A.

FIG. 7 illustrates a further embodiment.

FIG. 8 is a plan view of an embodiment having two independent radialzones.

FIG. 8A depicts a first embodiment of a dual port RF power source.

FIG. 8B depicts a second embodiment of a dual port RF power source.

FIG. 9 is an elevational view corresponding to FIG. 8.

FIG. 10 is an orthographic projection corresponding to FIG. 8.

FIG. 11 illustrates a further embodiment corresponding to FIG. 8.

FIG. 12 illustrates a modification of the embodiment of FIG. 8, in whichthe inner coil is a single winding of a conductor.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the appendeddrawings illustrate only exemplary embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

FIG. 1 depicts a plasma reactor in accordance with one embodiment. Theplasma reactor includes a processing chamber 100 enclosed by acylindrical side wall 102, a floor 104 and a ceiling 106. A vacuum pump108 evacuates the processing chamber 100. A workpiece support pedestal110 within the processing chamber 100 includes a workpiece supportsurface 112 for holding a workpiece 114 in facing relationship with theceiling 106. An array of process gas injectors 116 extend into theprocessing chamber 100 and are coupled to a gas manifold 118. A gassupply conduit 122 is coupled from a process gas supply 123 to the gasmanifold 118. Optionally, an RF bias power generator 127 is coupledthrough an impedance match 128 to an electrode 129 underlying theworkpiece support surface 112.

The plasma reactor of FIG. 1 has an inductively coupled plasma sourceincluding a coil antenna 140, an RF power generator 142 and an RFimpedance match 144 coupled between the RF power generator 142 and thecoil antenna 140. In the embodiment of FIG. 1, the coil antenna 140overlies the ceiling 106. The ceiling 106 is adapted to permit inductivecoupling of RF power from the coil antenna 140 into the interior of theprocessing chamber 100. For example, the ceiling 106 may be formed of anon-conductive material, or of a semiconductor material.

The coil antenna 140 has a low inductance, and its structure is bestseen in FIGS. 2-4, depicting an RF supply ring 150, an RF return ring152 and plural conductive lobes 154. The RF supply ring 150 is connectedto the RF impedance match 144 and the RF return ring 152 is connected toground, and are coaxial with one another. Each conductive lobe 154 is anelongate conductor having one end connected to the RF supply ring 150and its opposite end connected to the RF return ring 152.

Referring to FIG. 5, each conductive lobe 154 has a major axis 156 lyingin a radial direction and a minor axis 158 orthogonal to the major axis156. The major axis, in one embodiment, intersects an axis of symmetryof the RF supply ring 150. In the embodiment of FIGS. 2-4 there are sixconductive lobes 154, although any suitable number may be employed. Themajor axes 156 of the conductive lobes 154 are oriented at uniformlyspaced angles about the axis of symmetry of the RF supply ring 150, inone embodiment. The length L of each conductive lobe 154 along its majoraxis 156 corresponds to the diameter of the coil antenna 140. The widthW of each conductive lobe 154 along its minor axis 158 varies as apredetermined function of position along the major axis 156. Thepredetermined function controls the overlapping of areas enclosed byadjacent conductive lobes. This overlapping can vary with radialposition. For minimum inductance, such overlapping is minimized. Inorder to increase plasma ion density in chosen radial locations, theoverlap between adjacent conductive lobes is increased in the chosenlocations and minimized in others.

In the embodiment of FIGS. 2-4, the RF supply ring 150 and the RF returnring 152 are placed at different locations along the axis of symmetry,so as to be separated by a gap. For example, the RF supply ring 150 iscloser to the ceiling 106 than the RF return ring 152. The gap betweenthe RF supply ring 150 and the RF return ring 152 exceeds the thicknessalong the axis of symmetry of each conductive lobe 154. This feature canpermit neighboring ones of the conductive lobes 154 to overlap oneanother without contacting each other. In the embodiment illustrated inFIGS. 2-4, the RF supply ring 150 is of a smaller diameter than the RFreturn ring 152. In another embodiment, this arrangement may bereversed, so that the RF supply ring 150 may be of a larger diameterthan the RF return ring 152.

An example is graphically illustrated in FIGS. 6A and 6B. FIG. 6Adepicts radial distribution of plasma ion energy (solid line), having anM-shaped non-uniformity. FIG. 6B depicts the proportion of overlappingareas of adjacent conductive lobes 154 as a function of radial position.In FIG. 6B, the function or radial distribution of the overlapcorresponds to an inverse of the M-shaped non-uniformity of FIG. 6A. Theoverlap distribution or function of FIG. 6B compensates for (or nearlyeliminates) the M-shaped non-uniformity of FIG. 6A, resulting in auniform distribution (dashed line of FIG. 6A). FIG. 5A depicts oneexample of how the conductive lobes 154 may be shaped to maximizeoverlap between adjacent conductive lobes at the center and edge of thecircular zone of the coil antenna 140 and to minimize the overlap in anintermediate zone between the center and edge.

As depicted in FIG. 7, variable capacitors 400 may be individuallyconnected in series between the respective conductive lobes 154 and theRF return ring 152. Alternatively, the variable capacitors 400 may beindividually connected in series between the respective conductive lobes154 and the RF supply ring 150. As a further alternative, eachconductive lobe 154 may be divided into two sections at a break, andeach variable capacitor may be connected in series between the twosections of the respective conductive lobe 154. A controller 410 governsvariable capacitors 400 independently. By changing the individualcapacitances of the variable capacitors 400, the circumferential (e.g.,azimuthal) distribution of RF power may be selectively adjusted.

FIGS. 8, 9 and 10 illustrate a low inductance coil antenna havingconcentric zones, including an inner antenna 240 and an outer antenna241 surrounding the inner antenna 240. A dual port RF power source 200is governed by a controller 410 to provide separate control of the ratioof RF power levels applied to the inner and outer antennas 240 and 241.This ratio affects radial distribution of plasma ion density over aworkpiece underlying the coil antenna.

The inner antenna 240 of FIGS. 8-10 includes an inner RF supply ring250, an inner RF return ring 252 and plural inner conductive lobes 254.The inner RF supply ring 250 and the inner RF return ring 252 areconnected across a first output port 200-1 of the dual port RF powersource 200. The inner RF supply ring 250 and the inner RF return ring252 may be coaxial with one another in one embodiment. Each innerconductive lobe 254 is an elongate conductor having one end connected tothe inner RF supply ring 250 and its opposite end connected to the innerRF return ring 252.

Each inner conductive lobe 254 has a major axis 256 lying in a radialdirection and a minor axis 258 orthogonal to the major axis 256. Themajor axis 256, in one embodiment, intersects an axis of symmetry of theinner RF supply ring 250. In the embodiment of FIGS. 8-9 there are sixinner conductive lobes 254, although any suitable number may beemployed. The major axes 256 of the inner conductive lobes 254 areoriented at uniformly spaced angles about the axis of symmetry of theinner RF supply ring 250, in one embodiment. The length L′ of each innerconductive lobe 254 along its major axis 256 corresponds to the diameterof the inner coil antenna 240. The width W′ of each inner conductivelobe 254 along its minor axis 258 varies as a predetermined function ofradial position (i.e., position along the major axis 256). Thepredetermined function controls the extent to which areas enclosed byadjacent inner conductive lobes 254 overlap. For minimum inductance,such overlapping is minimized. In order to increase plasma ion densityin chosen locations, the overlap between adjacent inner conductive lobesis increased in the chosen locations and minimized in others.

In the embodiment of FIGS. 8-10, the inner RF supply ring 250 and theinner RF return ring 252 are placed at different locations along theaxis of symmetry, so as to be separated by a gap. For example, the innerRF supply ring 250 is closer to the ceiling 106 than the inner RF returnring 252. The gap between the inner RF supply ring 250 and the inner RFreturn ring 252 exceeds the thickness along the axis of symmetry of eachinner conductive lobe 254. This feature can permit neighboring ones ofthe inner conductive lobes 254 to overlap one another without contactingeach other. In the embodiment illustrated in FIGS. 8-10, the inner RFsupply ring 250 is of a smaller diameter than the inner RF return ring252. In another embodiment, this arrangement may be reversed, so thatthe inner RF supply ring 250 may be of a larger diameter than the innerRF return ring 252.

The outer antenna 241 of FIGS. 8-10 includes an outer RF supply ring251, an outer RF return ring 253 and plural outer conductive lobes 255.The radius of the outer RF supply ring 251 and the radius of the outerRF return ring 253 exceed the length L′ of each inner conductive lobe254, so that the outer rings 251, 253 surround the inner antenna 240.The inner conductive lobes 254 define a circular inner antenna zonewhile the outer conductive lobes 255 define an annular outer antennazone.

The outer RF supply ring 251 and the outer RF return ring 253 areconnected across a second output port 200-2 of the dual port RF powersource 200. In one embodiment, the outer RF supply ring 251 and theouter RF return ring 253 are coaxial with one another. Each outerconductive lobe 255 is an elongate conductor having one end connected tothe outer RF supply ring 251 and its opposite end connected to the outerRF return ring 253.

Each outer conductive lobe 255 has a major axis 257 lying in a radialdirection and a minor axis 259 orthogonal to the major axis 257. Themajor axis 257, in one embodiment, intersects an axis of symmetry of theouter RF supply ring 251. In the embodiment of FIGS. 8-9, there areeight outer conductive lobes 255 although any suitable number may beemployed. The major axes 257 of the outer conductive lobes 255 areoriented at uniformly spaced angles about the axis of symmetry of theouter RF supply ring 251, in one embodiment. The length L″ of each outerconductive lobe 255 along its major axis 257 corresponds to the outerdiameter of the outer antenna 241. The width W″ of each outer conductivelobe 255 along its minor axis 259 varies as a predetermined function ofradial position (i.e., position along the major axis 257). Thepredetermined function controls the extent to which areas enclosed byadjacent outer conductive lobes 255 overlap. For minimum inductance,such overlapping is minimized. In order to increase plasma ion densityin chosen locations, the overlap between adjacent inner conductive lobesis increased in the chosen locations and minimized in others.

In the embodiment of FIGS. 8-10, the outer RF supply ring 251 and theouter RF return ring 253 are placed at different locations along theaxis of symmetry, so as to be separated by a gap. For example, the outerRF supply ring 251 is closer to the ceiling 106 than the outer RF returnring 253. The gap between the outer RF supply ring 251 and the outer RFreturn ring 253 exceeds the thickness along the axis of symmetry of eachouter conductive lobe 255. This feature can permit neighboring ones ofthe outer conductive lobes 255 to overlap one another without contactingeach other. In the embodiment illustrated in FIGS. 8-10, the outer RFsupply ring 251 is of a smaller diameter than the outer RF return ring253. In another embodiment, this arrangement may be reversed, so thatthe outer RF supply ring 251 may be of a larger diameter than the outerRF return ring 253.

FIG. 8A depicts one embodiment of the dual port RF power source 200,showing its connection to the antenna. The dual port RF power source 200of FIG. 8A has a single RF power generator 205 coupled through an RFimpedance match 210 to the outer RF supply ring 251 of the outerconductive lobe 255. The dual port RF power source 200 includes ajunction 214 at which the outer RF return ring 253 and the inner RFsupply ring 250 are connected together. A voltage divider capacitor 215is connected between the junction 214 and ground. A variable capacitor220 is connected between the inner RF return ring 252 and ground. Thecontroller 410 governs the variable capacitor 220 in order to change orcontrol the ratio of RF power levels applied to the inner and outerantennas 240 and 241, respectively.

FIG. 8B depicts another embodiment of the dual port RF power source 200.The dual port RF power source 200 of FIG. 8B has a first RF powergenerator 242 coupled through a first RF impedance match 244 to theinner RF supply ring 250, and a second RF power generator 243 coupledthrough an RF impedance match 245 to the outer RF supply ring 251. Theinner RF return ring 252 and the outer RF return ring 253 are connectedto ground. The controller 410 varies the output RF power level of eitherthe RF generator 242 or the RF generator 243 in order to control orchange the apportionment of RF power to the inner and outer antennas 240and 241 respectively. This feature can provide control of radialdistribution of plasma ion density.

FIG. 11 depicts a modification of the embodiment of FIGS. 8-10, in whichvariable capacitors 402 are individually connected in series between therespective inner conductive lobes 254 and the inner RF return ring 252,and variable capacitors 404 may be individually connected in seriesbetween the respective outer conductive lobes 255 and the outer RFreturn ring 253. Alternatively, the variable capacitors 402 may beindividually connected in series between the respective inner conductivelobes 254 and the inner RF supply ring 250, while the variablecapacitors 404 may be individually connected in series between therespective outer conductive lobes 255 and the outer RF supply ring 251.As a further alternative, each inner conductive lobe 254 may be dividedinto two sections at a break, and each variable capacitor 402 may beconnected in series between the two sections of the respective innerconductive lobe 254. Similarly, each outer conductive lobe 255 may bedivided into two sections at a break, and each variable capacitor 404may be connected in series between the two sections of the respectiveouter conductive lobe 255. The controller 410 governs each of thevariable capacitors 402 and 404 independently. By changing theindividual capacitances of the variable capacitors 402, thecircumferential (e.g., azimuthal) distribution of RF power within theinner concentric zone may be selectively adjusted by the controller 410.By changing the individual capacitances of the variable capacitors 404,the circumferential (e.g., azimuthal) distribution of RF power withinthe outer concentric zone may be selectively adjusted by the controller410. The controller 410 may be able to control radial distribution ofplasma ion density by adjusting the RF power ratio between the inner andouter antennas 240, 241. At the same time, the controller 410 maycontrol azimuthal distribution of plasma ion density in the inner zoneby separately adjusting individual ones of the variable capacitors 402,while controlling azimuthal distribution of plasma ion density in theouter zone by separately adjusting individual ones of the variablecapacitors 404.

FIG. 12 depicts a modification of the embodiment of any one of FIG. 8 or11, in which a conventional coil antenna 300 replaces the inner coilantenna 240. The conventional coil antenna may include a singleconductor wound in a spiral or in a helix. Its size may be relativelysmall in order to limit inductance.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A coil antenna comprising an inner antenna and anouter antenna surrounding said inner antenna, wherein: (A) said outerantenna comprises: (1) plural conductive outer lobes extending inrespective radial directions with respect to a central axis, each ofsaid plural conductive outer lobes comprising an elongate conductorhaving a first outer end and a second outer end; (2) a first outerconnection to the first outer ends of said plural conductive outer lobesand a second outer connection to the second outer ends of said pluralconductive outer lobes, said first and second outer connections beingadapted for coupling to an RF power source; (B) said inner antennacomprises: (1) plural conductive inner lobes extending in respectiveradial directions with respect to the central axis, each of said pluralconductive inner lobes comprising an elongate conductor having a firstinner end and a second inner end; (2) a first inner connection to thefirst inner ends of said plural conductive inner lobes and a secondinner connection to the second ends of said plural conductive lobes,said first and second inner connections being adapted for coupling to anRF power source.
 2. The coil antenna of claim 1 wherein: respective onesof said plural outer conductive lobes extend radially from respectiveones of said first and second outer ends; and respective ones of saidplural inner conductive lobes extend radially from respective ones ofsaid first and second inner ends.
 3. The coil antenna of claim 1 whereinthe elongate conductor of each of said plural conductive outer lobes andof said plural conductive inner lobes follows a curved path comprising aloop.
 4. The coil antenna of claim 1 wherein: said first and secondouter connections are separated by a first gap distance and said firstand second inner connections are separated by a second gap distance; andthe elongate conductor of each of said plural conductive outer lobes hasa thickness along said axis of symmetry less than said first gapdistance, and the elongate conductor of each of said plural conductiveinner lobes has a thickness along said axis of symmetry less than saidsecond gap distance.
 5. The coil antenna of claim 4 wherein adjacentones of said plural conductive outer lobes at least partially overlieone another without contacting one another, and adjacent ones of saidplural conductive inner lobes at least partially overlie one anotherwithout contacting one another.
 6. The coil antenna of claim 5 wherein:adjacent ones of said plural conductive inner lobes overlap one another,and a proportion of overlapping areas of said plural conductive innerlobes varies as a first function of radial location; and adjacent onesof said plural conductive outer lobes overlap one another, and aproportion of overlapping areas of said plural conductive outer lobesvaries as a second function of radial location.
 7. The coil antenna ofclaim 6 wherein at least one of said first and second functionscompensates for a predetermined nonuniformity in radial distribution ofplasma ion density in a processing zone adjacent said coil antenna. 8.The coil antenna of claim 1 further comprising: a first RF power portcoupled to said first and second inner connections; a second RF powerport coupled to said first and second outer connections; and acontroller capable of governing a ratio of power levels of said firstand second RF power ports.
 9. The coil antenna of claim 1 furthercomprising first respective variable capacitors connected in seriesbetween respective ones of said elongate conductors of said pluralconductive inner lobes and one of said first and second innerconnections.
 10. The coil antenna of claim 9 wherein said one of saidfirst and second inner connections is connected to a return potential.11. The coil antenna of claim 1 further comprising second respectivevariable capacitors connected in series between respective ones of saidelongate conductors of said plural conductive outer lobes and one ofsaid first and second outer connections.
 12. The coil antenna of claim11 wherein said one of said first and second outer connections isconnected to a return potential.
 13. The coil antenna of claim 6 furthercomprising at least one of: (a) respective first variable capacitorsconnected in series between sections of respective ones of said elongateconductors of said plural conductive inner lobes; (b) respective secondvariable capacitors connected in series between sections of respectiveones of said elongate conductors of said plural conductive outer lobes.14. A coil antenna comprising an inner antenna and an outer antennasurrounding said inner antenna, wherein: (A) said outer antennacomprises: (1) plural conductive outer lobes extending in respectiveradial directions with respect to a central axis, each of said pluralconductive outer lobes comprising an elongate conductor having a firstouter end and a second outer end; (2) a first outer connection to thefirst outer ends of said plural conductive outer lobes and a secondouter connection to the second outer ends of said plural conductiveouter lobes, said first and second outer connections being adapted forcoupling to an RF power source; (B) said inner antenna comprises asingle conductor wound in a coil shape.
 15. The coil antenna of claim 14wherein said single conductor of said inner antenna is wound about saidaxis of symmetry.
 16. The coil antenna of claim 14 wherein respectiveones of said plural outer conductive lobes extend radially fromrespective ones of said first and second outer ends.
 17. The coilantenna of claim 14 wherein: said first and second outer connections areseparated by a gap distance; and the elongate conductor of each of saidplural conductive outer lobes has a thickness along said axis ofsymmetry less than said gap distance.
 18. The coil antenna of claim 17wherein adjacent ones of said plural conductive outer lobes at leastpartially overlie one another without contacting one another, andadjacent ones of said plural conductive inner lobes at least partiallyoverlie one another without contacting one another.
 19. The coil antennaof claim 14 further comprising: a first RF power port coupled to saidfirst and second outer connections of said outer antenna; a second RFpower port coupled to said inner antenna; and a controller capable ofgoverning a ratio of power levels of said first and second RF powerports.
 20. The coil antenna of claim 14 further comprising respectivevariable capacitors connected in series with respective ones of saidelongate conductors of said plural conductive outer lobes.